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What is currently the longest commercial flight in terms of distance?
Flightradar24 has announced few hours ago, that Qantas QF7 is taking off for its 13,804 km long flight and named it the longest commercial flight (by distance).
Is that correct? I was more than sure, that I heard few years ago, that current record belongs to Singapore Airlines, that has at least one commercial route of a little bit more than 14k kilometers.

Today I read this article on Aviation Week on ultra-Long Range routes, and I remember this conversation, I just wanted to contribute!

According to Wikipedia, the flight you are referring to, SQ21, got discontinued in November 2013. The runner up, still from Singapore Airlines, SQ37, got discontinued one month earlier.
The same page reports that QF8 is the current holder of the title of "longest non-stop flight" and has the same destinations as today's QF7 as of February 2017 Qatar Airways flights 920/921, covering 14,534 km (7,848nm / 9,031mi) from Doha, Qatar (DOH/OTBD) to Auckland, New Zealand (AKL/NZAA), beats QF8 by about 395 NM.
If we instead include 1-time-only flight by a commercial airliner, the longest one has been flown in November 2005 between Hong Kong and London, as a demo by Boeing.
The title of absolute longest flight without refueling is instead held by a non-commercial flight done in an aircraft specifically built for the purpose.

Your answer (and my question) is about commercial, repeatable route. Just a small notice, from the article you linked to, about a longest, world record, single passenger flight: "On 9 November 2005 a Boeing 777-200LR, dubbed the Worldliner, completed the world's longest non-stop passenger flight. It traveled 21,602 kilometres (11,664 nmi) eastward, as opposed to a normal westward routing for that sector, which is much shorter at 9,647 kilometres (5,209 nmi), from Hong Kong to London, in roughly 22 h 22 min" .

@trejder you're right, I updated the answer including also the absolute longest flight

Wait - the november 2005 flight WAS NOT a commercial, passenger-carrying flight. (It was not a charter or anything.)  It was just a demo by Boeing, carrying only Boeing staff, journalists etc.  Added an article link.  Note that the wiki page confusingly reads "commercial aircraft" but mixes in that list one-off non-paying-passenger flights, with various demonstrations and tests

@JoeBlow Well it was a commercial aircraft, as distinct from a military, prototype or experimental aircraft. It just wasn't in commercial service that day.

Hey David - sure, it was a commercial aircraft, not a commercial "FLIGHT" - note the question title.  I was just pointing out it's a little confusing. The question is very much about longest "flight" in the sense of airline routes (not experimental aircraft, etc).

At this moment the longest route is EK448/449 between dubai and Auckland, start services 2days ago. But it will soon be surpassed by UA1/2 between Singapore and San Francisco

I heard also that Emirates is planning a non-stop flight from Dubai to Panama City.

Today I read this article on Aviation Week on ultra-Long Range routes, and I remember this conversation, I just wanted to contribute!

@Him SQ now also flies SFO-SIN direct. They're also planning to restart the SIN-NYC service next year and, if I understand correctly, also start a direct service to LAX.

According to this CNN article, the record has been broken because of the COVID-19 outbreak:

On March 14, French airline Air Tahiti Nui flew the longest ever scheduled passenger flight by distance -- transiting 9,765 miles across the world from Papeete, in Tahiti, French Polynesia, to Paris' Charles de Gaulle airport.
This one off milestone was a direct consequence of the coronavirus-induced US travel restrictions.
This route usually involves a scheduled stopover at Los Angeles International Airport (LAX). But when an airplane stops over in the US, all passengers must alight the aircraft and proceed through US Customs and Border Protection before they're allowed to advance on with the next leg of their journey.
Current restrictions rendered this part of the journey untenable, so instead, flight TN064 just carried straight on, departing at 3 a.m. local time from Papeete airport and arriving in Paris at 6:30 a.m. local time on March 15.

This is a scheduled flight, but a one time deviation from the schedule (though the situation may continue to be like this in the near future).

"Going forward, the flights are planned to use Vancouver as a transit point." (gcmap.com) Sounds like it was a one time deviation.

Agreed, I'm not sure this counts as a regularly scheduled commercial flight.

Air India 173 flies over Pacific some times. It often travells 9500 miles+ in 15 hours.
On 21st October 2016, it flew 10,204 miles (16,480 km).
Source

True, though most of the others are measuring "longest flight" by the shortest-path distance between the city pair rather than by actual distance flown, since all airline flights can significantly change course from one day to the next (due to weather, winds aloft, turbulence, TFRs, traffic, etc.) For measuring longest airline route, distance between the city-pair is a more useful measure, as it doesn't change from day to day (well, not unless you take continental drift or the effects of large earthquakes into account.)

@reirab LOL! I think that even "continental drift or the effects of large earthquakes" doesn't have that significant influence into airline route, after all! :>

@trejder They might, if said continental drift or earthquakes coincide with certain other types of geological activity. :-)


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Why hasn't there been a case of an airliner breaking apart and crashing due to turbulence in the past 40-50 years?
Most of the breakups involved clear air turbulence as well, the type that still can't be detected today.
Examples like:

Boeing 707 (BOAC Flight 911) - which crashed near Mt Fuji

BAC One-Eleven breakup in the 60's

Fairchild F-27B broke up in turbulence over Alaska

Fokker F-28 broke up in turbulence after flying into a ''microburst'' in 1981

De Havilland Comet
broke up after flying into a storm in the 1950's.

Mishap investigation reports and engineering lessons learned?

Agreeing with @MichaelHall, but it would be interesting to know what were these improvements, and from which accidents there were devised.

@mins Seconded.

I don't disagree, I guess my point is that the question could be improved per @mins' comment.  Can you cite a turbulence induced inflight breakup?

@MichaelHall BOAC Flight 911, BAC-1-11 breakup in the 60's, F-27 broke up in turbulence over Alaska, F-28 broke up in turbulence after flying into a ''microburst'' in 1981, De Havilland Comet broke up after flying into a storm in the 1950's. Those are just off of the top of my head.

So maybe fold that info into your question?

Not sure if this answers your Q - link to a BBC article on crashed that reshaped plane design.

Flew on Comet in 1964. By then pretty much sorted. As a family flying from Singapore to RAF Lynham we were very aware of the Comet's reputation. As a child, it did'nt really bother me but as we boarded the flight, I can still hear my mother with exclamation "oh my god".

The BAC One-Eleven that crashed on a test flight on 22 October 1963 did so because the aircraft entered a deep stall, peculiar to rear engine T-tailed aircraft. Not due to CAT.

Durability of Modern Aircraft
Modern Day airliners (or even those built in the 1970s like DC-10) are built to withstand really high G-Forces. Despite the certification requirements for civil aircraft are +2.5G and -1G.
Incidents involving high G-Forces
Federal Express (FedEx) Flight 705: Highlighted parts of the Incident include, the aircraft (a DC-10) being steeply pitched up 15°, then using ailerons to roll the aircraft to 140° attitude (way past the certification of maximum 60°).
The aircraft was flying upside-down, with its engines at full-power, this caused the mach tuck effect (the airflow over the wings reached the speed of sound).
During Landing, the aircraft was 16,000kg overweight, really high, really fast. Captain Sanders made really sharp turns (again exceeding the DC-10's certified limits) and landed the DC-10 just as it feels in a normal landing. Despite it being damaged, it was flyable and is in service with FedEx as of May 2022

1986
2015

China Airlines Flight 006: On  19 February 1985, High altitude upset and 30,000 ft dive due to spatial disorientation after failure of Engine 4. The failure of Engine 4 lead to asymmetrical thrust and the aircraft began to yaw. This was compensated by the Autopilot using the ailerons. Due to the autopilot using ailerons instead of rudders (The rudder could not be deflected by the Autopilot on that aircraft) to compensate for asymmetrical thrust, the aircraft kept yawing despite completely turning the yoke (100% aileron deflected)
The crew realized the aircraft was banking, the crew was disoriented, they thought the autopilot was not performing corrective actions (rather, not working) and hence disengaged it. As soon as A/P was disengaged, the ailerons no longer compensated the yaw and the PF neither gave rudder or aileron inputs causing the aircraft to bank instantly leading to an uncontrollable descent.
When on ground, it was noted that the landing gear doors broke apart, a lot of the vertical stabilizer was missing from the aircraft, and the G-Forces of about 5G (for 2½ minutes) caused the wings to permanently bend (but not break) but still it landed safely. Despite such damage, repairs were done and the aircraft was used till late 2014.

Attitude
Damage Caused

Such incidents prove that, airliners built today are capable of withstanding high G-Forces without disintegrating mid-air. But windshear (microbursts) is still a concern.
The LLWAS and Onboard Windshear Alerts
The above incidents prove that aircraft built today (the DC10, MD11, A320/50/80) are highly durable and so explains most of the incidents of mid-air breakup you mentioned. But, they are prone to another problem. Windshear during approach. There have been multiple incidents in the 80s (even in the 90s) where windshear has caused aircraft (even a 737) to smash into the runway (or ground) while landing (United Airlines Flight 585)

Downdraft
Windshear

Source:
https://safetyfirst.airbus.com/wind-shear-an-invisible-enemy-to-pilots/

But, Modern day airliners have Windshear Detection systems onboard and inform the crew about a potential windshear well before getting into it. The airports are equipped with LLWAS (Low Level Windshear Alert System) whose alerts can be communicated by ATC to the crew.
This concludes that, due to LLWAS and onboard windshear detectors and also highly durable aircraft makes disintegration due to turbulence a really, really rare scenario.

Potential Windshear Alert
Currently in Windshear

Fun Fact: The De Havilland Comet which you mentioned was so much less durable that, its first version (Comet 1) used to disintegrate due to explosive decompression caused by metal fatigue after a year from manufacture. It had square windows which caused high stresses in the corners due to repeated compression/decompression cycles on each takeoff/landing causing the fuselage to fail. BOAC (British Airways) Flight 781 and South African Airways Flight 201 both crashed in 1954 due to the design and was grounded as it became evident that all Comets were going to explode mid-air. However, De Havilland rectified this issue and launched the Comet 1A (not sure of the model number) but meanwhile the Boeing 707 was launched and airlines preferred Boeing 707 over the Comet. The Comet's last model was Comet 4C which retired in 1997.

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Is the ILS only used for landing when weather conditions are poor?
Either I'm missing something, or this article clearly suggests, that pilots are using ILS for landing only in bad weather conditions. Which could lead to an assumption, that in perfectly clear weather they're landing always or nearly always in visual.
Even though I'm completely new to aviation, I was always sure that ILS is used on every landing procedure, no matter if weather conditions are good enough to do it visually. ILS landing seems to be faster, more precise, more safe, and thus -- in my naive opinion -- should be used whenever it is available (i.e. whenever ILS is commissioned and operational).
What am I missing?

Visual approaches are still sometimes made when weather conditions are favourable - if the ILS fails, for example, you'd want to know that your pilot regularly practices visual approaches!

And when a pilot is unable to fly a Visual in clear conditions, bad things happen

I think the point you're missing is the difference between navigation systems (include ILS) and manual/autopilot. You can land following the ILS glideslope/ localizer either piloting manually or using the autopilot.

@JonStory "Sometimes", right. But cited article clearly states (at least I understand it like that), that ILS is used only in foggy / bad weather, which may lead to an assumption (at least it leads me), that ILS in clear weather isn't used at all or even using it is prohibited. That was strange to me.

@ManuH No, I don't confuse that two things. I'm asking, because in my opinion cited article clearly states, that ILS is used only in foggy / bad weather, which was strange to me.

@trejder - it's just ambiguous wording. ILS must be used in IMC, and it may be used at any other time. Airlines/airports may also have policies that state it should/shouldn't be used at other times, and in VMC it comes down to those policies and the choice of the pilot/controller.

@trejder to complete what Jon Story says in its comment, the article does not state anything about ILS use in good weather but that it must be used to guide the aircrafts in foggy conditions.

I don't think that the article suggests "pilots are using ILS for landing only in bad whether conditions". it simply says that under foggy (i.e. in bad weather), ILS is used as visual approach is not possible.
The use of ILS is at the discretion of pilots with ATC permission. The ILS localiser and glideslope can still be used as a reference during the visual approach (even if the landing is not ILS). The decision is the pilots' as the visual indicators and glideslope may not match.
While it is true that ILS is much more precise and safer, you are forgetting the human factor- the pilot may want to 'fly' the aircraft rather than letting the computer do it.

Even if it's a CAVU day I like to dial in the ILS frequency (when available) just as an extra sanity check.  Nobody wants to be the guy that lands at the wrong nearby airport...

@BrianKnoblauch Like those guys from Czech Airlines, in common joke (joke?), which confused Prague with Vienna, because they confused week days (they're landing in Prague every Tuesday and Friday and in Vienna in all other days), right? :>

The relevant question is not whether or not the ILS system is used at all, but when the pilot switches from ILS to visual.
At the beginning of an ILS approach, the ILS signals function as a navigation aid, helping to make sure that the aircraft is lined up correctly with the runway from a longer distance out than the runway itself would be visible clearly enough to the naked eye even in good weather.
At some point during the approach, the runway will be clearly enough in sight that the pilot can actually land the plane visually. The latest point at which this is allowed to happen is the "decision height" -- if the runway is not reliably in sight by then, a go-around must be executed.
The lower the decision height, the larger demands are placed on the accuracy and reliability of the ILS equpiment -- losing the ILS signal due to interference for a few seconds while still six miles out is much less critical than losing the signal for a few seconds right before touchdown.
It is possible (if all the involved equipment is maintained and certified to high enough standards) to have a decision height of 0 feet so the ILS/autopilot flies the aircraft all the way to touchdown, but doing this requires all other aircraft moving around on the airport surface to observe extended safety distances to the runway to minimize the risk of radio signals bouncing off their fuselages and sending a blind-landing aircraft astray. This can have bad effects for capacity, so in normal operation landings are done with a nonzero decision height.
In fact, most airports don't spend the additional maintenance cost needed to certify their ILS systems for zero decision height ("category III" ILS operation), or do it only for some of their runways. In case of bad weather at busy times, some flights will just have to divert to other airports.

This incident is good demonstration what can happen if a plane follows ILS below the minima without the protected areas being observed.

Thanks for a detailed answer. However, you have focused on explaining (to a newbie like me), how ILS works and how pilots operates during landing with or without ILS. While I was actually asking, if my conclusion from cited article (that using ILS in clear weather is something odd, as in my opinion that article suggests, that ILS is use only in bad weather) is true? So it was rather, who is wrong (article author or me, making wrong assumptions), not: how does ILS works.

@trejder: Using the ILS in clear weather is not odd -- it is practically always being used for the beginning of the approach. What would be odd (or at least require a particular reason to happen) is to let the autopilot fly all the way to touchdown in clear weather, because (a) the safety precautions needed to make this safe harms airport capacity, and (b) even with all the respect distances being observed, there's still a lot more redundancy in visible light and two pairs of Mk 1 eyeballs than there is in a narrow-band ILS signal with one antenna.

Pilots don't (usually) do autoland, i.e. having autopilot follow the ILS all the way down to the ground, in good weather.
Using autoland requires that no other aircraft or vehicles pass in designated protected area around the ILS transmitters, because they could disturb the signal and cause the aircraft to land outside the runway like this Singapore Airlines B777 in Munich when they tried autoland in good weather for practice and did not tell ATC/ask them for cooperation.
Because these procedures restrict operations, the tower controller won't normally apply them if the weather is good enough to use visual or CAT I approaches in which the final part of the approach must be flown by pilot.
However, visual approach just means that the pilot must have visual reference to the runway. They are still free to use any additional reference and pilots will often tune the ILS (or the FMS will even tune it automatically when the approach procedure is selected) to verify they are approaching the intended runway and for cross-checking the glide-slope.

It's my understanding that sometimes the crews do still need to autoland in good weather just for currency, but, for the reasons you mention, they should cooperate with the tower in order to do so. I'd assume they try to avoid doing that at busy times at busy airports, though.

I'll have to dig up the reference, but our FOM required us to use any and all available electronic guidance on an approach.  That meant on a visual approach, if there was a localizer (and glideslope), it was tuned as well.  That was also a good sanity check to make sure you were lined up with the right runway and at the right airport.

@reirab, yes, they do. It is what the Singapore crew was trying to do. And unfortunately there are not many airports that would have a CAT II or III ILS and not be busy.

@casey That makes sense. Helps to avoid really embarrassing mistakes.

@JanHudec Yes, they'll usually be busy airports, but they won't usually be busy 24/7. Even Atlanta is pretty quiet late in the day.

@JanHudec I'm confused by your answer because the linked article disagrees with you. According to the article they used ILS because of weather conditions and it also states that they were cleared for ILS approach.

@JulianeHolzt, the weather was below visual, but above CAT I minima and they were cleared for ILS with CAT I minima, which does not permit auto-land. However, they opted to auto-land without telling the controller.

ILS is sometimes used in clear weather, it's at the discretion of the pilots and company policy (if it's commercial). Most pilots I know would prefer to fly a visual approach because flying is what they do - what's the point if the airplane does all the fun stuff? Even in instrument conditions pilots will often manually fly the ILS approach rather than using autopilot to keep up their skills.
ILS is only one out of several approach systems which exist. There are NDB, VOR, surveillance radar, and now GPS approaches which can be used.
I fly light aircraft from an airfield which has no approach aids whatsoever, when I fly from there it's always visual as it cannot be anything else.

As a student pilot that isn't IFR trained yet, we always fly visually and don't use the ILS. How I land depends on if it's at a towered airport, non-towered airport, or if I'm coming in from 10 miles out or if I'm already in the vicinity of the airport.
TL;DR what you're missing is that most pilots start out as VFR only and the last thing you want is to be distracted by things inside the cockpit like GPS devices, tuning an OBS, loads of instructions from tower controllers, etc. Every pilot needs to able to fly the airplane without the assistance of an ILS or a GPS.
General Final Approach
On final approach, if flying visually, we power up if the aiming point is descending in the windscreen and we power down if it's climbing. Power is altitude, but may require a corresponding pitch adjustment.
If we get slow, we nose down and vice-versa if our speed is a little high. We adjust power as necessary based upon our pitch adjustment.
Non-towered Landings
These are fairly straightforward. We enter the pattern for the runway we've chosen at a 45-degree angle to the downwind leg. We then turn base then final and end up 1-2 NM out on final, ideally 500-600 feet AGL and descending at 500 FPM. Again, this is a purely visual landing where we're focusing our aiming point in the window and trying to keep it steady by using power.
Towered Airport Landings
Most of the towered landings I've done the controller has said to advise when 2 miles out and told us to aim for a base leg for runway X. We then fly a decently long base leg, turn final ideally at 500-600 feet AGL, and land just like at non-towered airports.
The controller could instruct us to do a 10-mile straight in, enter on the downwind, enter on base, etc., but it just depends on traffic and controller load.
I have yet to be instructed to use the ILS as a visual (VFR) pilot and I wouldn't want to b/c as a student, it's very helpful to learn ground references, ground track, crosswind correction, etc. without the assistance and distraction of in-the-cockpit devices.

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What is the difference between an aeroscraft and an airship?
I stumbled across this site when I was searching for info about airships. They have main three products - aerocraft aeroscraft, airship and aerostat. Aerostat is clear for me somehow. But what is the key difference between aerocraft aeroscraft and airship?

@mins: Actually the page says Aeroscraft; it does not mention it without ‘s’ anywhere.

Aeroscraft is their brand name. It does not mean anything generally.
The general terms are:
"Aircraft" (but not "aerocraft") means any machine capable of sustained flight in the atmosphere.
"Aerostat" is a lighter-then-air aircraft. That is it has large shell filled with gas with lower density than air and uses buoyant force on that for lift. This includes balloons, hot-air or gas filled, tethered balloons and airships.
"Airship", or "dirigible" is an aerostat equipped with propulsion that allows it to move on it's own (as opposed to balloons that are tethered or carried by wind).

So "Aerostat" is a fancy name for a hot-air balloon?

@FreeMan: No. Hot-air balloon is one kind of aerostat. Hydrogen or helium filled balloons and airships are also aerostats.

Ah, so "Airship"/"Dirigible" is a sub-class of "Aerostat" that indicate they have propulsion and are steerable. I guess that's what your last line says. I'll just go back to napping now... :/

@FreeMan Balloons and Airships (including Dirigibles) are all aerostats (the term refers to how they stay aloft - aerostatic buoyancy). Some may have propulsion (airships), some may be free-flying without propulsion (balloons), and some may be tethered to the ground by cables (tethered balloons). In the US when people refer to "an aerostat" they're frequently referring to some kind of tethered balloon, like the radar balloon off the Florida Keys, but that's an imprecise use of the term.

There are 2 kinds of lift able to be produced, aerodynamic and aerostatic. Planes and helicopters are aerydynamic because they need to move in order to create the lift, balloons and dirigibles are aerostatic because they create lift by simply being lighter than the air around them. This is why we call them aerostats. I wonder why we don't call the other class aerodynes...

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How can ADS-B replace primary radar when FlightRadar24, using ADS-B, is so inaccurate?
I read the following on Flightradar24.com's 'How it works' page.

The percentage of aircraft equipped with ADS-B receivers is steadily
increasing though, as they will become mandatory for most aircraft
around the world by 2020. When mandatory, ADS-B will replace primary
radar as the primary surveillance method used by ATC.

Sure, that sounds lovely, but it it seems to me that a large portion of aircraft do not actually use GPS, but instead rely on inertial navigation systems or similar. Of course, the ATC doesn't need millimeter precision, or even an accurate position within a kilometer, but whenever I check flightradar24, there is quite a large bunch of airplanes reporting their position with offsets up to a few kilometers - if I were to judge by the ADS-B data, I would report a major emergency every few minutes.
How will ADS-B actually reliably replace primary radar? Will GPS have become mandatory by 2020? Or is Flightradar24.com just being optimistic?
(Side note: I know that redundancy is very popular in the aviation world, but I don't think you can call something 'replaced' when primary radar is still used for a large portion of the flights)
Edit: some parts of some answers seem to be concentrating on update rate / transmission losses and errors. For clarification, I included a screenshot (which I could literally take the second I looked at fr24.com, indicating how often this happens).

At the time this picture was taken, the 36R runway was in use for landing ('yellow' aircraft top right just landed). The selected aircraft is however happily taxiing in a field (pattern matching 36R's adjacent taxiways). This is not caused by update rate or transmission losses (i.e., this can't be old information, the plane was never in that field). Transmission error also seems out of the question, since the entire track is a smooth line (errors sometimes do happen, but are not 'hidden' on fr24 - a large 'jump' in the track can be seen sometimes). So either the plane is reporting a 'wrong' position (how is this handled by ATC?) or fr24 somehow messes up the data (how is that possible?

The central premise here is stated incorrectly: what has been observed is NOT that ADS-B is inaccurate. What has been observed is that FlightRadar24 has glaring inaccuracies.  There is an assumption being made that ADS-B is the reason for these observed issues, and that is at best an unsupported assumption, and possibly even an unfounded one as well.

See this question for a discussion of FlightRadar24's issues which occur separately from the inherent accuracy of a full-up ADS-B system. Particularly, read the second answer, which is even more on-point here than the first.

TLDR

How will ADS-B actually reliably replace primary radar?

It will not.

Will GPS have become mandatory by 2020?

Strictly speaking no, effectively yes.

There are so many problems with that quote from FR24 that I don't know where to start.
Let's take it apart and analyse it sentence by sentence.

The percentage of aircraft equipped with ADS-B receivers is steadily increasing though

Totally irrelevant. An ADS-B receiver allows reception of ADS-B signals from other aircraft. These can be used for example to create a Cockpit Display of Traffic Information (CDTI) or augment TCAS. The number of aircraft with this so called ADS-B IN is increasing, but the equipage percentage is probably below 2% at the moment.
If you are concerned with using ADS-B operationally for ATC then airborne receivers are not of interest. Instead you should be looking at the transmitter side.

[ADS-B receivers ...] will become mandatory for most aircraft around the world by
2020.

ADS-B receivers are not mandatory in any foreseen regulation yet. ADS-B transmitters are / will become mandatory in many parts of the world. The EU and USA rules mandate ADS-B in 2020.

When mandatory, ADS-B will replace primary radar

ADS-B is not intended to replace primary radar. As a new addition to the surveillance technology mix, it will not replace any other technology in the short term.
ADS-B is a form of cooperative surveillance, meaning that the aircraft or vehicle that is supposed to be tracked needs to cooperate by transmitting the ADS-B signal. Primary Surveillance Radar (PSR) on the other hand is a non-cooperative surveillance technology; it works by detecting signals reflecting off the target of interest. PSR can detect aircraft that do not transmit any signals. Since PSR and ADS-B are fundamentally different in their working, they have fundamentally different use cases. ADS-B will not replace primary radar and never was intended to.

will replace primary radar as the primary surveillance method used by ATC

Primary Surveillance Radar is not the primary surveillance method used by ATC in many parts of the world. Since PSR only gives azimuth and range measurements essential information like aircraft identification and altitude is missing. Therefore Secondary Surveillance Radar (SSR) is the primary surveillance method used for ATC since it provides altitude and identity in addition to azimuth and range measurements.
Secondary radar, like ADS-B is a form of cooperative surveillance; it relies on the aircraft replying to an interrogation from the radar. ADS-B can and will replace a number of secondary radars, but not all of them.

Now for the rest of your question.

it seems to me that a large portion of aircraft do not actually use GPS, but instead rely on inertial navigation systems or similar.

Indeed a large number of aircraft do transmit their position on ADS-B that is based on a non-GPS source, usually from the INS. INS can not provide the accuracy and integrity that GPS can provide and therefore you see large position errors on FR24. The fact that the position is not to be trusted is reported in the ADS-B message in the quality indicators, but unfortunately FR24 doesn't use this information.
All the ADS-B mandates require that the system is able to provide high integrity data, which effectively means that GPS is required as position source, although theoretically other means are possible as well. Therefore in 2020 all aircraft subject to the various ADS-B regulations will have to be GPS equipped.

Of course, the ATC doesn't need millimeter precision, or even an
accurate position within a kilometer,

ATC does not need millimeter precision (GPS wouldn't provide that anyway) but they do need position accuracy within a kilometre and sometimes within about 10 meters (e.g. runway entry stop bar monitoring). ADS-B  with appropriate GPS source will be able to provide that.

Uh, the whole 'off by miles' part ;) So, how does ATC handle an aircraft reporting itself to be a few miles off course? (especially near airports, since I guess that's where a few miles actually matter)

Data from those aircraft isn't used. If ATC uses ADS-B position data, then the aircraft has to be certified for ADS-B and the transmitted quality indicators need to be indicating that the position integrity and accuracy are above a certain level (which varies based on the ATC environment). The aircraft from your screenshot does report INS position and at the same time reports that the data is not to be trusted.

Isn't it as simple to say, that ADS-B is just to enhance and provide meta-data to the already existing systems?

"ADS-B with appropriate GPS source will be able to provide that." - hmm, and then the first enemy nation to bring down GPS will crash the world's economy even more.

The linked page also states that as part of ADS-B,

Aircraft gets its location from a GPS navigation source (satellite)

This strongly suggests that any aircraft using INS position only (perhaps with DME/DME updates only at present) would be required to add a GPS receiver as part of the ADS-B upgrade.
While no installed + approved + certified aviation equipment will ever be as cheap as a basic handheld Garmin from your local sporting goods store, the big cost of ADS-B seems likely to be from things other than the GPS piece of it. In E-GPWS systems, the GPS receiver can be dedicated to just that system, without incurring costs of integrating it into the FMC, so it may be possible to do likewise with ADS-B.  This would probably reduce costs somewhat, although I don't know how long INS/DME RNAV will be allowed, given the move toward "Next-Gen" everything in ATC.

Thanks for the answer. However, I disagree with your 'strongly suggests', since evidence strongly suggest the reported position is off by miles. Do you think that 'Next-Gen' aircraft will have compulsory GPS then?

@sanchises - Can you provide a reference saying that ADS-B will use position information from sources other than GPS? Every piece of technical documentation I've come across states that GPS is used to determine aircraft position and flight vector, meaning that even ADS-B Out kits will have to feature GPS receivers, if the aircraft itself doesn't already. FWIW, if you're seeing reported positions that seem to be off by a few miles, it's not happening due to ADS-B's limitations; I've first flown ADS-B-equipped aircraft nearly 10 years ago and position accuracy really is as good as claimed.

@habu I know that GPS is perfectly accurate enough, as well as ADS-B's position transmission capabilities - hence my assumption that these planes were apparently using INS. Any idea why then the position is off by miles?

@sanchises - Couldn't tell you for sure, but my first guess would be some sort of glitch or limitation in the FR24's code; that's why I was asking for references, thought maybe you'd come across something I hadn't.

First of all, what you see on Flightradar24 is not the same as what air traffic controllers see. ADS-B requires a ground station. The FR24 receivers are placed sporadically in peoples homes - far from the sophistication of the proper, more powerful, ATC stations. The coverage on FR24 is often sloppy but the controllers screens are fine.
ADS-B is already mandatory in Australia above FL290. As you can see here, ADS-B brings many benefits - none more so than on page 5 of that booklet. There is a diagram there that compares existing radar coverage to ADS-B coverage. Only a tiny bit of Australia is covered by radar. Radar is very expensive to install and maintain, and doing so in the desert cannot really be justified. But ADS-B provides virtually complete coverage of the country at cruise, perfect for the huge amount of Asia/Europe traffic. But most of this traffic will not appear on FR24 because they don't have many receivers out in the middle of nowhere.

This post, although interesting, does not answer my question in any way. I'm not asking about coverage, but position accuracy, which is independent of the receiver (either the receiver does or does not receive the airplane position).

True, I was more focusing on showing you that real ADS-B is more accurate than Flightradar24, but I have not actually compared it to radar. I will do some research and come back to you (unless someone else can answer beforehand)

As stated in other answers, the receivers used by FR24 is not the same receivers as ATS use. In a comment you indicate that this is irrelevant:

"I'm not asking about coverage, but position accuracy, which is
  independent of the receiver (either the receiver does or does not
  receive the airplane position)."

This is however relevant indeed. Poor reception can lead to a poor update rate (only some of the broadcasts received), leading to old data being displayed. Also, you don't know what signal/data processing occurs between the receiver and you on FR24, and what processing would occur at the ATSU.
The performance specification for ADS-B is laid out in ICAO doc 9871 (which unfortunately comes with a bit of a price tag). Unfortunately, I don't have access to 9871, however, it is reflected in its various national ratifications.
The following are quotes (my emphasis) from Annex 2, Part B in EU 1207/2011 (which most likely is quite harmonized with FAA/other regs, at least within the scope of this question):

§6 The primary data source providing the data items in point 3(h) and
  (i) shall be at least compatible with GNSS receivers that perform
  receiver autonomous integrity monitoring (RAIM) and fault detection
  and exclusion (FDE), along with the output of corresponding
  measurement status information, as well as integrity containment bound
  and 95 % accuracy bound indications.

...

§10. The total latency of the horizontal position data (the data items in point 3(h) and (i)) shall be equal to or less than 1,5 second in 95
  % of all transmissions.

...

§11. The uncompensated latency of the horizontal position data (data item in point 3(h)) shall be equal to or less than 0,6 second in 95 %
  of the cases and shall be equal to or less than 1,0 second in 99,9 %
  of all transmissions.

Point 3(h) and (i) are respectively: geodetic position and horizontal position quality indicators
From this, we can deduce that the accuracy of the position input will be bound to GNSS/RAIM, and that the latency of the position data will be in the order of a second, or less. (This however says nothing between time to updates).
Certification requirements of a GNSS-system is controlled by various regs, depending on the installation (ABAS/RAIM, SBAS, GBAS etc) and many of them once again have price tags, but just to get an idea of the precision an ADS-B system could expect we can look to former FAA TSO-129a (GPS/RAIM), in which it states that the equipment 2D position error must be contained to 0.124 nm for oceanic, en-route and terminal ops, at 95% confidence (about 230 meters).
The actual ADS-B equipment requirements are more involved than this (and the possible precision/accuracy of GNSS with a suitable augmentation system (SBAS/GBAS) is much higher), but these figures should at least give a ball park idea of its precision.
It should be mentioned that the article you link to claims that primary radar is the primary surveillance method used by ATC. At least for most civilian ATC units, this is incorrect: Secondary surveillance radar (SSR) is the primary surveillance method, with primary radar sometimes being available as a supplement to SSR-data. As SSR is dependent on a transponder reply from the target aircraft, while a primary radar is able to detect objects without a transponder, ADS-B can not replace the supplementary function of a primary radar per se, but it can replace/extend the function of SSR.
Both FAA and Eurocontrol has stated ADS-B out will be required in certain airspace from 2020. In Australia it has been required above FL290 already since 2013.

This is an older question, but I wanted to add a couple points that I didn't see addressed in any of the answers provided.
As thoroughly explained, Flightradar24 (FR24) provides users with a depiction of near-real time (NRT) civil aviation activity using the aircrafts' ADS-B-out data transmissions. However the website has the capability, albeit it limited, to determine the position of aircraft utilizing a Mode-S transponder without ADS-B out transmissions.
Mode-S transponder transmissions contain a range of data depending on the type and features of the system, but without ADS-B capabilities, they do not  provide an aircraft's position. The key piece of data that is included is what is commonly referred to as the "Hex Code". A hex code can be defined and decoded in a bunch of ways, but in it's most simple form it is a unique number that is assigned to a specific aircraft. Yes, there are ways to change it in the avionics shop, but if normally only done if an aircraft is deregistered in one country and registered in another.
Every country which operates under the International Civil Aviation Organization (ICAO) is allotted a block of these codes for use by aircraft registered in their country. So when a Mode-S transmission is received and processed, the system processing it is able to identify the specific aircraft which transmitted the signal. If the the system has at least 4 receivers in an area where all are able to determine that this unique aircraft is transmitting a signal, and the precise positions of those receivers are accurately measured, and those positions are integrated with allowing for the system to use logic algorithms written by people WAY smarter than me, you are able to do something called multilateration (MLAT).
For those of you thinking "that sounds a lot like triangulation", you're exactly spot on. MLAT is a fancier more technically advanced form of triangulation. Historically one would use direction finding equipment which would literally point to the source of a transmission. Get enough arrows pointing, you can figure out an approx. location. With MLAT, the system instead utilizes the extremely small differences in time it takes for the signal to reach each receiver to determine the distance the aircraft is from the receiver. Get enough distances, you can determine a location.
The important point to realize is that MLAT has the potential to be very precise when determining aircraft positions. However, in order to achieve this type of precision the receiver antenna positions and system set-up requires a far higher level of scrutiny and quality control that even the best FR24 receiver host could provide. With certified ATC systems, even antenna cable quality and run lengths are taken into consideration.
So if you consistently see aircraft positions being depicted inaccurately by FR24 in a certain area, check the signal details to determine if MLAT is being used.
The other point is that FR24 overlay maps may have errors or inaccuracies with respect to the geo-referencing. Even a small error can manifest as a significant position indication.
So hope that helps even though it's a little stale!!!

What people are missing here is FR24 is NOT for ATC operation but secondary data source to be used as 'non critical use' by anyone interested. Technology for ADS-B in/out itself is perfectly accurate when operated under certification guideline of aviation authority. In reality FR24 web service or other crowdsourcing data provider never 'guarantee' 100% accuracy for their data. Also FR24 website show intentionally delayed data by couple of minutes.Website user  access level also have different accuracy on different time of day between FREE and Paid account. Also if you carefully look on 'settings' of FR24 web interface you will find option for 'Estimations value in minutes' which sometime show aircraft position entirely by estimation of schedule, last received data etc in case of no real data received for any particular scheduled aircraft.
There are other limitation for FR24 data viewing which might give wrong impression. But if ever ADS-B become primary for ATC operation, they will have their own setup for that purpose but ATC operator will never guide aircraft based on screen what you and me are watching on open Internet. TCP data like HTTP/Web traffic will never use on time-critical operation. Switching mode data network is there for time critical network. Even your VoIP traffic normally avoid TCP for voice transmission between two party.
ADS-B is NOT Flightradar24.COM, but Flightrader24.com use ADS-B for sourcing their data. Deciding ADS-B technology by watching Flightrader24.Com is like trying getting experience of traveling on Airbus A380 while traveling on Boeing DC-10:))
Edit adding more info: In nutshell if we compare all other technology available for avation industries for accuring position data at this moment ADS-B is on top of list. If you don't want to beliave FR24 website, try FAA document issue 2010 on this PDF Document. By using FR24 site you got wrong impression about ADS-B. Actually this is very much accurate in operation when use as per regulation guideline and equipment setup both on aircraft and control tower is proffational grade. Just think of it, people now preparing for riding driverless on highways runing hundrudes of kilomiter/hour with much proximity than aircraft would never come close (Even on a parking space). Both position data source is GPS sattelite indeed. Current operating prosidure used guiding allmost all avation traffic is product of Warld War II era. Equipment might become sophisticated but structure is same. You can have more details on ADS-B here in Wikipedia Article.

This doesn't answer the question. I never said that ATC would use FR24, I was just wondering about the accuracy of ADS-B. The delayed information is no longer the case (that is an outdated FAA thing). You don't pay for accuracy but for access to more data (like wind and temperature aloft as calculated by the aircraft).

Also, 'perfectly accurate' is never true. Only as accurate as the aircraft's navigation, which is quite good in the case of GPS, but rather poor for some older airplanes which haven't bothered to link their GPS to the ads-b transceiver. For example, old KLM Fokker planes which never flew in Australia anyway, and are now retired.

What I wanted to say is FR24 use ADS-B and provide near accurate data as referance for using non-critical public use at this time. As tracking technology ADS-B is most accurate compiring other available technology in operation. Editing answer to add some more description. Thanks

@Sanchises:  "I never said that ATC would use FR24, I was just wondering about the accuracy of ADS-B".  True, but your assumptions about the inaccuracy of ADS-B is based solely on your observations of a free web app.  I would suggest you get friendly with your local ATC facility and ask if they will show you their displays.  You might be impressed...

@MichaelHall As the other answers show, a local ATC facility does not use ADS-B as the primary source of data (and also not primary radar as FR24's website suggest), but secondary radar ('pinging' a transponder). I stand by my observations concerning ADS-B even if they are from a web app, as there is no degree of 'accuracy' involved in reading a digitally broadcast position - either you receive a valid ADS-B position report, or you have some bits wrong and the checksum doesn't add up, and you drop the report....

... From this I concluded that, surely, ATC wouldn't rely solely on these ADS-B self reports. It seems that indeed ATC uses secondary radar and does not (yet) rely on ADS-B self reporting for all aircraft, and will not in the near future.

"...or FR24 somehow messes up the data (how is that possible?)"  I am not an expert on digital data format conversion and transmission, but I know what happens when I make a photocopy of a photocopy of a photocopy...  I think this is the most likely cause of what you are seeing, even if it is an incomplete explanation.  Or maybe it isn't even FR24, but the guy with an ADS-B receiver on his dining room table feeding the WWW.  Good luck finding an answer that satisfies your curiosity!

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


text:
Fuel consumption database using ICAO codes?
I will preface this by saying I do not have any level of knowledge in Aerodynamics - I'm a freelance journalist and so apologies for the poor level of general understanding.
I'm attempting to create a visualisation that would depict CO2 produced per time in the air per flight, but have not been able to find any comprehensive dataset for average values of fuel consumption per hour per aircraft model.
My goal is to calculate an approximation how much "additional" CO2 is being emitted by delays.
Most flight calculators I've been able to find give an estimate per person, or per person which tends to evoke distance (https://www.icao.int/environmental-protection/CarbonOffset/Pages/default.aspx / https://co2.myclimate.org/en/flight_calculators/new)
Therefore the constants given in the methodologies tend to reflect a fuel consumption per distance in their datatables - which given the fact that the distance travelled overall remains "the same" even if the plane is late doesn't really tell much.
I've attempted to look through the questions here but best thing I've been able to find is a forum post from years ago that isn't comprehensive (here)
I get the feeling I'm missing a source glaringly obvious by not using the correct terminology....
Thank you in advance for your help.

Your linked post looks pretty comprehensive to me.  What major types and models are missing?  If there are specific makes and models not included that you feel contribute a statistically significant amount I would think you could research them individually, or ask here.

At the risk of stating the obvious, I think what you need is the typical cruise speed of each of the models listed in the other post.  From this you could create a spreadsheet that would give you fuel burn by distance rather than by time, and you could plug in distances of any ICAO airport identifiers to compute the burn on a typical flight between two points by each aircraft type.  Does that get you any closer to what you are looking for?  The math itself is pretty easy, but let us know if you are unsure how to do what I am suggesting.


text:
Is there any automated go-around procedure?
In Wikipedia's description of one of air crashes (probably Thai Airlines, if I recall correctly), I found a remark saying something like that: "Crew decided to go around, but apparently decided to undertake this procedure manually".
I'm surprised. "Apparently"? Making go around manually is something odd? Do we have any automated (autopilot-based) go around procedures or techniques in common use? I was always more than sure, that procedures like go around or touch-and-go are always made manually by a pilots.

Wikipedia says "On Airbus planes pushing throttles to TOGA detent [Take-Off/Go Around] does all regarding flight path and speed"

Just to clarify, all instrument approaches terminate at a hold after the missed approach segment. If you don't land and reconnect the autopilot, it will take you through the missed approach and enter the hold (assuming your autopilot is advanced enough) once you fly past the touchdown waypoint (or perhaps the missed approach point, I'm not all to familiar with automation details).

For go-around, several things have to be done in the moment the decision is taken:

Apply full power (obviously),
disconnect autopilot glide-slope mode,
disarm spoilers and retract them if they are extended,
disarm autobrakes and
in some aircraft reduce flaps.

Modern airliners, and some smaller aircraft, have a method for doing all of these at once via the TO/GA button. On Airbus aircraft it is done by just advancing the thrust levers to the TO/GA (full power) position, but on Boeing aircraft and many other types it is a separate button on or near the thrust levers.
So if the crew "apparently decided to undertake this procedure manually" it means they moved the thrust levers forward and started changing the other things. And probably missed one of them.
Notes:

I am not sure whether lateral mode remains under automatic control or not and it probably depends on aircraft as some can connect autopilot to only roll or only pitch and others don't. Pitch control is manual immediately after go-around in either case.
The last flap setting usually adds a lot of drag and a little lift. In some types the drag is too large and the flaps have to be retracted to the last usable position. On other aircraft they can be used in any position and have to be left were they were to avoid loosing lift.

Normally on a "go around" the pilot flies the airfield traffic pattern and attempts to land again, usually after a visual approach. This is done manually as far as I know. The "missed approach" which is a published instrument procedure, is described in the aircraft's FMS (for an FMS equipped aircraft) and can be activated by the crew and flown automatically.
Since I am not a professional this is as afar as I know. More details (about rules regulations e.t.c.) can be provided by others. But unless you provide more details about the accident, I think that little can be explained for the case.

A "go around" on an instrument approach implies flying a missed approach, not a traffic pattern.

@falstro What happens when an instrument approach is terminated and becomes visual? Is it a go-around then or the published missed? But the real question for me is: Is there an automated way to perform go-around in terms of traffic pattern? Or it should be manually executed, even if that requires continuous autopilot input from the pilot? That's what I'm missing and frankly makes me think of withdrawing my answer...

The go-around and the missed approach are two different things. The go-around itself is the process of terminating the landing attempt and configuring the airplane to position for another approach. The missed approach following an instrument arrival is a described lateral and vertical path, a set of instructions carried out to actually get back to the initial approach fix (IAF). The missed approach follows the go-around, but the terms really mean different things. Patterns are usually for VFR flights and not made after IFR approaches.

The visual part of an instrument approach is still an instrument approach, thus "going around" means you fly the missed approach for that instrument approach (it hasn't terminated, it's just on the visual segment of the approach). It's a little more blurry if you fly a visual approach (still IFR) as they don't have a published missed approach (AFAIK). If you cancel IFR and do a VFR approach, then of course you do the normal traffic pattern. As for the rest, all approaches terminate at the hold after the missed, the autopilot can probably take care of that once you make the decision not to land

@falstro In the US, if you are IFR and are cleared for a visual approach (I think that's what you meant by a "VFR approach"), then it isn't blurry.  There is no missed approach procedure to fly in this situation and you are expected to enter the traffic pattern/coordinate with ATC.  See this question.


text:
Do I need to call Clearance Delivery for IFR Practices when taking off from a D airport?
I have the following question:
If I am going to practice IFR approaches under VFR, do I need to contact clearance delivery for that or it's enough to contact ground and state my intentions?
I would deffinetly contact clearance delivery on a C airport, but I am not so sure about a D airport. For sure I will contact clearance delivery if the practices were under IFR with a filled IFR plan.

Just curious, but if you are VFR at a class C airport what would you ask them for when you call clearance delivery?  Squawk for flight following I guess?

Well, in class C you need to call clearance and they will give you instructions based on what you requested. So yes, they are going to give you an squawk code for the departure, if you stay there you will keep it, if you depart and continue VFR you can cancell and squawk VFR

Thanks.  I flew out of class C for years, but always IFR.  I have entered C plenty of times VFR, but never left that way.

When flying VFR from a Class D airport, there is no need to contact clearance delivery to practice IFR approaches. You may, on the other hand, have to call clearance delivery if that is the local procedure for all departures at that airport. This is not very common at a Class D airport. If a clearance is necessary for VFR flights at a Class D airport, most often it is given through ground control. The US Chart Supplement and/or the ATIS will indicate if it is otherwise.
Normally, a simple indication to ground control of your intentions will suffice. This lets them know how to prepare for your return with the rest of their traffic flow and provide separation. This would be a demonstration of common and professional courtesy. It is not mandatory.
You can request a practice approach once airborne and inbound back to the airport without notifying ATC before your departure. As a matter of fact, it is very common to request a practice approach at any class of airport at a distant destination when making your initial call and position report prior to entering the airspace. Be prepared to be denied the approach if the airport is extremely busy during VMC.

Yes, exactly what I was thinking of. I was planning to take off, do some maneuvers and after that contat approach for the IFR practices.I just wanted to confirm that there is no need to contact clearance delivery, if I want to have flight following from the take off, thanks!

@IvanParra Also keep in mind that if a runway has multiple approaches from different directions you may not get practice approaches from the opposite direction of traffic. My airport will never allow opposite direction approaches but the airport 25 miles away will almost always allow them, although you may have to hold for departing traffic.

You only need radar services, usually you pick this up with approach if in flight or you can ask ground or tower for flight following before takeoff, then "Request practice approach NDB/DME-B, VFR".
At many class D ground is the same as clearance delivery; even if it is two or three frequencies they only have one controller most of the day that answers clearance, ground, and tower.

Nice answer. Flight following is always a good idea. In the US, it is not mandatory. You could fly VFR without a flight plan, flight following, radar services, or any other type of ATC services other than approach and landing clearances. If you are about to enter C or D airspace, you have to have two-way communication with ATC. But, there are Class D airports without radar services (visual only). You can’t even get flight following from most visual Class Ds. In VMC, you can just request the practice approach directly from tower. You don’t even make a request at a non-towered airfield in VMC.


text:
Who to contact when inbound to adjoining class deltas?
Given two class delta airports that share a boundary, say you wanted to transition through one to land at the other.  Who would you call first?  The transitioning controller, or the destination controller?
(skyvector.com)
Example: approaching KOXR from the east for landing through KCMA, VFR w/o flight-following.  Could the KOXR controller clear me through KCMA airspace?  Being conservative, I suppose I could call both but wonder if I'd have time for a frequency change before entering KOXR.

Why not just call SoCal approach on 120.4 and let them handle the transition?

I used to fly into the LA area frequently and only rarely did they not give me flight following. I never had them drop me on the way into the LA area. If you get flight following from your departure airport they probably won’t drop you because they’d rather know what you are going to do than have you do unexpected things.

You should contact the tower of the Delta you intend on transitioning through/entering first. Tell them you want to transition through their airspace. If they clear you to enter their airspace you can request a frequency change while still inside the first delta and contact the second. This is the same procedure as say departing a Delta that abuts a Bravo then taking a Bravo transition. You could also ask for a handoff to the second delta from the controller at the first.
For example I fly out of KPNE often a Delta abutting a Bravo, if I want to fly south through the bravo before take off I can call the tower with something along the lines of,

"Archer 123PB holding short at 33 ready for departure to the south to KWWD.
  Can I get a bravo handoff as well?"

An example in your case might be

Camarillo tower, Piper 123PB 5 miles to the east looking to transition through your airspace to KOXR

The tower may respond with

Piper 123PB approved as requested maintain 1500 ft. hold course and overfly the field

Then as you are a bit more east (most likely past the field in this case)

Camarillo tower requesting frequency change

Tower

Frequency change approved

You can then call OXR with

Oxnard tower Piper 123PB 3 miles to the east inbound for a landing at OXR with information {current AITS}

This is going to happen fast and you will need your frequencies in hand for this. You can ask the first tower for a handoff which they may orchestrate but you should be prepared for both options. If you have some kind of a pre-programmable radio this is a great example of where queueing up your frequencies will help.
Your other option (in this specific case) weather permitting you could overfly KCMA above 2000 ft. since the delta ends at 2000 Ft. and simply request entrance to the second delta. In this case you never actually enter the first delta and no clearance is required.
In some cases, there are prior agreements that may not match the charts for adjacent airspaces.

+1, although to be completely accurate you don't need a clearance for class D so the controller is more likely to respond "Piper 123PB, approved as requested, maintain 1500' etc." As soon as you hear your callsign, you're good to enter.

@Pondlife you are correct, I have heard it both ways but I will adjust since your quote is a bit more accurate.

"You can ask the first tower for a handoff"  I haven't heard of such a thing.

Please do not EVER say the leading "Kilo" over the air to any controller who works in the Lower 48, and if you're talking about local airports (certainly within 30-40 miles of where you are, probably farther) it's easier and quicker to just say the name instead of the identifier.

@David If both towers have a radar display (not guaranteed with class D) and you’re on a discrete code, they can almost certainly do a handoff. In fact, they’ll generally do it without you even asking.


text:
What is the absolute velocity change through compressor+burner?
I thought that the velocity going through a turbojet increased as it went through the compressor and the burner. Intuitively, if the pressure/enthalpy increases, so would the velocity, right? I came across with this image, though:

As you can see, this image says that overall velocity actually decreases at the exit of both compressor and burner.
What is the logical explanation behind this? Shouldn't the compressing/burning effect actually accelerate the flow?

The law of conservation of energy dictates that the pressure and velocity of a fluid are inversely proportional. So the higher the pressure the lower the velocity and vice versa.

I left temperature out of that, but it still applies. Compressing air reduces its velocity, accelerating air reduces its pressure.

@TomMcW, there is an anomaly in the graph though. Mostly is shows the speed decreasing as the pressure increases and vice versa, but in the combustor stangely both pressure and velocity decrease, while temperature remains constant.

@JanHudec That is odd. Neither this graph, nor  this one nor this one show that, so I'm wondering if the graph has an error. Dunno tho.

I also see now that he specified at the exit of the compressor and burner. So i think i misunderstood his question

@TomMcW, well, at the exit of compressor it does decrease, because the combustor is wider than the compressor, so the velocity must decrease from conservation of mass. According to the graphs you found, the velocity increases throughout the burner (combustor), so that appears to be an error in the graph.

Related: http://aviation.stackexchange.com/q/11744/11953

@JanHudec At the ext of the compressor, when the veloiity drops rapidly should there not be a corresponding s pike in pressure and/or drop in temperature? Only one of the four graphs shows that

@TomMcW, hm, you are right, that does seem strange.

@TomMcW Total Pressure and Total Temperature do not change with velocity. The static pressure does increase at the compressor discharge. A diffuser is used to recover the pressure without causing backflow.

@TomMcW: The cross section is not constant - after the compressor it expands to slow the flow down. That section is on purpose called the diffusor, and there pressure stays about constant while the speed drops. The relationship between speed, pressure and temperature only holds when the volume (or here: the cross section) stays constant.

The most basic conserved value here is that in steady state, the rate of mass flow across any given cross-section is a constant. This is a function only of density, area of the cross-section, and velocity perpendicular to the cross-section, and the density is a function only of pressure and temperature (OK, also chemical composition, given the burning of the fuel.)

@TomMcW it’s best not to think about energy conservation here as you are taking energy out of the flow with the turbine and sending it to the compressor which then does work on the flow. As a result if you just consider the flow moving through the compressor you find it’s energy mysteriously increases as the compressor does work on it, adding energy to the flow.

The flow enters the compressor at approximately Mach 0.4 to 0.6 in order to ensure a high mass flow with still subsonic compressor blade speeds. Compressing the air allows to reduce flow speed, and between compressor exit and combustion chamber is a diffusor to reduce flow speed even more. Why? To ensure a high degree of combustion! The longer the reactants remain in the combustor, the better. And large combustors are heavy, so they are kept as short as possible.
As soon as the air heats up, it picks up speed. In that respect your graph is not quite correct (or it assumes a rapidly widening cross section through the combustor). But the general relation between pressure and speed is right: While no energy is added or removed, the sum of pressure energy and kinetic energy of the flow is constant. While both the compressor and the combustor add energy and the turbine removes it again, the nozzle is such a region of constant energy. Here the remaining pressure of the flow is converted into speed.

This image that TomMcW posted in the comments is a much better representation of what is going on.

First, looking at axial velocity only is only half of the story, through the compressor and especially through the turbine the airflow can be more circumferential than axial. In fact the point of the stator vanes and turbine nozzles (turbine vanes) is to turn the airflow to give the proper angle of attack on the blades.
The velocity drops in the compressor because the flow area doesn't decrease as quickly as the air density increases, by design. This increases the efficiency of the compressor and keeps the air moving slower at the compressor exit.
The flow area increases a lot at the compressor discharge going into combustor, this is to slow the air down for better combustion and to ensure the static pressure on the outside of the liner is higher than on the inside. In the combustor, two things happen to increase the velocity, first a massive amount of heat is added, decreasing the density requiring a higher velocity and the area gets smaller, also increasing velocity, both can be calculated with the continuity equation. You also get a small amount of mass increase through the combustor from the fuel.
Now at the exit of the combustor you have the Stage 1 turbine nozzles, which accelerate the airflow to Mach 1, and generally with a very sharp circumferential angle. The air then slows and is straighten as the turbine blade extracts work. This is repeated through each stage of the turbine, although it will not achieve Mach 1 again.
Also remember Mach 1 changes throughout the engine with total temperature.

Your answer was equally as good as that of @PeterKampf, thank you both for your insights!

authors: محمود طباطبایی


text:
Maximum/Lowest achievable expansion ratio (generally) in a turbine?
Let's think about a turboprop with a single shaft. The turbine moves the compressor and the propeller.
My question: what is the "limit" to the expansion ratio achieved by the turbine?
If we call $5$ the air that is exiting the turbine, and $4$ the air that enters the turbine, then the expansion (or compression) ratio is defined by: $$\pi_t\equiv \dfrac{p_{05}}{p_{04}}<0\Longleftarrow \left(\textit{Because air is expanding}\right)$$
But for a non-ideal, single-stage axial turbine this ratio can be related to $T_{04}$ (that is, the temperature at the burner exit) by: $$\pi_t=\bigg[1-\dfrac{W_t}{c_p\eta_tT_{04}}\bigg]^{\gamma/\left(\gamma-1\right)}\quad \text{where}\quad \gamma\approx 1,35>0$$ where $W_t$ is the specific energy extracted from the fluid by the turbine.
On the one hand, as $T_{04}$ increases, $\pi_t$ will increase as well.
But on the other hand, I think we are interested in $T_{04}$ as high as possible BUT $\pi_t$ as low as possible, which comes into direct contradiction right? (Please, correct me here if I'm wrong).
What is the compromise solution between $T_{04}$ and $\pi_t$? If we make $T_{04}$ too high (which is thermodinamically convenient) then $\pi_t$ will be higher than its optimal value, and the other way around.

EDIT:
$T_{04}$ has itself a structural/thermal limit (if it goes too high, it can ultimately melt and/or generate way too high pressure on the turbine blades until it breaks). I think this defines a maximum value for $\pi_t$, right? Although knowing the maximum $\pi_t$ I think doesn't make sense, because we want to minimize it.


text:
What are the advantages, disadvantages and current state of affairs for text-based ATC?
This answer has a rather interesting (and quite lengthy) discussion, on a concept of using text-based rather than voice-based communication between ATC and the cockpit. As it turns out, the FAA has some plans for that already as mentioned by @reirab. Being a simmer rather than a pilot, I've always found text-based ATC quite convenient (it provides a direct reference to your own instructions as well as instructions to others on your frequency), but the discussion in the linked answer showed some pilots opposed to such a system. My question is twofold:

What are the advantages and disadvantages of such a system?
What is the current state of affairs of this concept? Are there specific methods of implementation across various instances - e.g., FAA, EASA, ICAO or other regulatory bodies - and do aircraft (systems) manufacturers anticipate such systems, or would perhaps a software update suffice on modern aircraft?

Edit: Perhaps it's better to concentrate mainly on IFR traffic, since their procedures are more formal (spacing, landing sequence etc. are decided by ATC) and things like voice call-outs (e.g., the nice voice that calls you a retard in an Airbus also calls for a go-around) are available at no extra cost.

This may be better to split into two questions.

@fooot I considered that, but I thought the answers would probably be very similar (people adding regulations to their advantage/disadvantage list, people adding advantages/disadvantages to their regulation overview) and the advantages/disadvantages might get too opinion-based without cross-checking with the current state of affairs.

The answers so far seem to address the advantages/disadvantages, so the state of affairs in implementation/regulation will probably be a separate answer.

Regarding your edit - the voice callouts are the plane speaking to the pilot. They are still for the pilot to choose what to do, and to communicate that as required to ATC. I don't see the relevance here? I also disagree that IFR is "more formal" - VFR communications are also prescribed and formal.

@Dan Urgent (or perhaps all) ATC messages addressed at the plane itself could be called out by the plane itself (and acknowledged by a nice button labelled "Acknowledge" or similar) to reduce radio clutter since obviously, something went wrong for ATC to order a go-around. I'm not talking about pilot-initiated go-arounds, although that could easily be linked to the TO/GA button, again reducing workload. (Note that I'm advocating this technique strongly to spark a debate, not because I'm too naive to see the downsides)

@fooot Fair enough - do you think I should split up the question now or see where it goes?

You can decide since it's your question, but to me the answers so far seem to suggest your second question would be good to ask separately.

There is one aspect that could be developed in answers, related to AF447. From the BEA report page 199: "The crew was not able to use the ADS-C and CPDLC functions with DAKAR Oceanic. If the connection had been established, the loss of altitude would have generated an alert on the controller’s screen".

Thnx for the award. I just added a bit of detail, if you like to know more, just post a comment and I see what I can add. I need to research and bit but for the next couple of years I will be involved with datalink anyway so it is time well spent.

I'm a newbie pilot, but I wanted to give the obvious example that crossed my mind when I read your question. In the UK GA world we have to, for example, call up to say we're on final approach. Finals can be a pretty busy time for the pilot, and at any kind of busy GA airport you're likely to have something along the lines of the following discussion:
(Note in my example, my airport has an "Information Service" rather than full blown ATC, but nonetheless - the example stands)

Me: "G-AB Finals, Runway 16"
Tower: "G-AB, Runway Occupied"
Me: "Continuing, G-AB"

This is me telling the controller that I'm aware I can't actually land, but I'll continue flying my approach path down towards some kind of decision height - somewhere around 300'. If I hear nothing again, I'll go around.

Tower: "G-AB, Land At Your Discretion"
Me: "Landing, G-AB"

I now continue my landing, except I'm not happy with my approach so I now declare a go around

Me: "G-AB, Going Around"
Tower: "G-AB, Roger"

Remember that all of the above is going on while I'm setting flaps, keeping on the glideslope, watching the runway and maintaining my airspeed.
Now also, imagine the alternative:

Tower: "G-AB, Land At Your Discretion"
Me: "Landing, G-AB"

So I now have clearance to land. I carry on, there's a bit of a crosswind and I'm working hard. I cross the airport boundary and I'm concentrating on the runway.. Unbeknown to me, a small dog/child/whatever has broken loose and is charging across the grass to the runway:

Tower: "G-AB, Go Around, I say again, Go Around"
Me: "Going Around, G-AB"

In less than 2 seconds the danger is gone, and the tower know now that I've understood and acknowledged. Now in no world am I ever going to be looking down at a screen during that moment - even if a message came through and beeped/bonged or whatever - my concentration will be on landing. Now you might argue for a specific "Go Around" call/button or whatever - but how many of those different things will we end up with. I simply can't imagine a situation where I won't need my headset on and radio tuned.
I'm not saying any of this stuff will never come in, but pressing a button to talk and listen is much more favourable to typing or reading something during critical phases of flight. At best, I can imagine it complementing some of the more mundane messages and perhaps Ground Controllers.
An ATIS text system might be nice, and I imagine routing and similar will be useful - but I can't imagine it taking over voice completely. Sometimes you just can't beat talking - there's a reason people still e-mail the line "I'll call you".
Beyond that - during those radio comms every aircraft in the vicinity now knows that G-AB is on Finals. This is incredibly important to GA traffic operating in VFR because pilots work hard to keep a mental picture of whose where - not to the degree the tower does, but if someone is following me but doesn't have visual (Very common - planes are surprisingly hard to spot), or loses visual, they know exactly where I am and what I'm doing.
The aircraft that's just touched the threshold also knows that control are aware the runway is occupied and hasn't just cleared an aircraft to land.

Finals?  How many are there? ;)

@Simon With my flying, at least 3 per landing ;)

@Dan Reminds me of the many, many Report final final version.pdf files on my hard drive...

The main advantages of text based ATC over voice based communication are

Earlier messages can be recalled
Messages can be printed
Messages can be read by automated systems, allow clearances to be loaded into the FMS
No inaudible messages due to radio static noise or simultaneous transmission
Time saving and frequency congestion reduction when transmitting and confirming position reports, which otherwise can take up to 20-45 minutes on an HF frequency to wait one's turn and then transmit on a noisy frequency.

Some disadvantages are

time delay for urgent messages
not able to bring across emotions, e.g. when giving a critical instruction to avoid disaster
since the communication is private, other parties can't listen in and don't  know what is going on: loss of situational awareness.

This makes text based ATC unsuitable for the TMA and CTA. However in busy/large enough route sectors, the disadvantages are not an issue since there are rarely time critical messages and traffic situation awareness is of less importance.
Detailed information can be found in the ICAO Global Operational Data Link document.
Text based ATC was developed as part of the Future Air Navigation System (FANS) and  is known as Controller-Pilot Data Link Communications  (CPDLC). It has been in limited operation since the end of the 1990s. Due to technical problems the uptake has been slow initially.
Whilst the system is far from perfect today, its use is increasing. CPDLC is in use in En-route airspace over Europe and over the North Atlantic (NAT).
See also this answer on the implementation phase on the NAT.
From 2018, all IFR flights above FL285 in Europe have to be equipped with CPDLC.

The main concern with text ATC is the loss of situational awareness.
When a controller issues an instruction, and the pilot responds, everyone else on that frequency also hears what's going on.  This is a major part of maintaining situational awareness and text based solutions, to my knowledge, have not yet come up with an alternative.
Situational awareness is most important when things are busy which also co-incides with the times when reading or writing text is more difficult.
Until these problems are solved, I can't see text replacing radio any time soon.  It may well augment it, for example by passing routing clearances which in general are not useful to others in maintaining awareness.

Unfortunately sometimes crippled by the fact that several languages are permitted for IFR instructions (besides English at least French, Spanish, Russian, not sure what else) and any language is permitted for VFR information. So in many places it is common that the controller talks to different crews in different languages and situational awareness goes down the drain (and when there is an incident, investigators always suggest to stick to English, but nobody ever mandates it).

At latest since the invention of IRC we found ways to also distribute text messages to multiple participants, just like voice communication.

@yankee So all pilots under the same control should be reading all of these text messages?  It's easy, with practice, to listen to the radio calls that are important to you.  E.g. if I'm on the ground, I'll probably ignore requests to taxi from a terminal at the other end of the field but one from the terminal I am at will definitely register.  This can be done whilst still maintaining lookout, scan etc.  I'd need to see convincing evidence that the same is true for text messages.

A very different answer to my first. While there's something of a catalogue of errors in this, the following audio recording is a clear display of why situational awareness is always a good thing to have:

In the recording, Flight 1448 accidentally finds themselves on the Active Runway. Despite realising their mistake and speaking up, the controller (for whatever reason) simply asks them to hold while giving take off clearance to another aircraft on the same runway. Sensibly, after hearing what's going on, the second aircraft opts to hold instead of taking off.

I give you the deadliest aviation accident caused by (amongst others) radio clutter and unclear voice communication, or this almost-incident caused by misheard callsigns. Also, in your video example, the pilots can just see the takeoff clearances on their screen: just like there is a crewmember monitoring the radio, there will be a crewmember monitoring the ATC screen. I think your other answer is better.

@sanchises You misunderstand - it was the aircraft WITH takeoff clearance who'd overheard the issue and decided to hold. It' s a fair rebuttal - but I feel you're playing devils advocate somewhat.

Guilty as charged

In the US, DataComm is coming. First to Clearance Delivery in the busier airports and soon to EnRoute aircraft. The pros and cons are a lot to discuss. More than a two beer topic.
The intent of DataComm is mostly for commercial aviation with appropriate equipment, not for the private or small business user. Eventually, the plan is to have the digital clearance uploaded automatically to the onboard nav systems. The advantages are manifold; reduction in missed readbacks, speedier reroute acknowledgements, reduced frequency congestion.
I don't believe there will a time when voice comm is completely replaced just due to the huge number of variables in an operational environment. Just like the final approach example that was given. For frequent routine communications though, it can't come soon enough!


text:
What is the difference between "Standard Metric Level" and "Altitude" in ICAO flight plans?
What is the difference between "S" and "M" in flight plan altitudes?
The FAA's ICAO flight plan documentation says:

Standard Metric Level in tens of meters, expressed as S followed by 4
  figures (for example, S1130), or
[...]
Altitude in tens of meters, expressed as M followed by 4 figures (for
  example, M0840)

I don't know what "Standard Metric Level in tens of meters" means exactly? How is that different from "Altitude"?

Does this answer your question? What is the difference between "flight level" and "altitude"?

The terms asked about here aren't used in that question.

@RalphJ Flight level and Altitude have exactly the same meaning, the only difference is the unit (metres vs. feet)

@expeditedescent - I think you meant to say, Standard Metric Level and Flight Level “...have exactly the same meaning, the only difference is the unit (metres vs. feet)“.

@DeanF. No, what I mean to say is that flight level is called flight level and altitude is called altitude, no matter if you are using feet or metres as the unit

Standard Metric Level is the metric equivalent of the foot-based Flight Level. It is what you get when the altimeter is set to QNE, i.e. above the Transition Level, and read in meters.
Metric Altitude is the metric equivalent to the foot-based Altitude. It is what you get when the altimeter is set to QNH, i.e. below the Transition Altitude, and read in meters.
These are actually the official ICAO systems, with the foot-based systems declared a "temporary" alternative.

In the FAA's ICAO flight plan documentation, it states the following:

Cruising level (maximum 5 characters)
  ENTER the planned cruising level for the first or the whole portion of the route to be flown, in terms of:
  Flight level, expressed as F followed by 3 figures (for example, F085; F330), or
  *Standard Metric Level in tens of meters, expressed as S followed by 4 figures (for example, S1130), or
  Altitude in hundreds of feet, expressed as A followed by 3 figures (for example, A045; A100), or
  Altitude in tens of meters, expressed as M followed by 4 figures (for example, M0840), or
  for uncontrolled VFR flights, the letters VFR.

Flight Level and Standard Metric Level are synonymous terms for the same value. Flight Level is measured in imperial feet. Standard Metric Level is measured in metric meters. Either, whether represented in feet or meters, is the distance at which an aircraft flies above the atmospheric level of standard pressure (Pressure Altitude). This is the Indicated Altitude shown on your altimeter when the Kollsman window is set to 29.92 inches of mercury (or the metric equivalent). In the US, Flight Level is what you would use at and above FL180.
Altitude, whether represented in feet or meters, is the distance at which an aircraft flies above the standard datum line of the average level of the sea (Mean Sea Level). This is the Indicated Altitude shown on your altimeter when the Kollsman window is set to the local atmospheric pressure (if it is accurate). In the US, altitude is what you would use below 18,000 feet MSL.
Depending on atmospheric pressure, there will be a transition space between 18,000 feet MSL and FL180.

The question was about "Standard Metric Level", not "Flight Level".

@HiddenWindshield - How are Standard Metric Level (represented in meters( and Flight Level (represented in feet) not, essentially, the same thing? The question does not ask for the difference between Standard Metric and Flight Level. The OP already understood that per his original, unedited, question. The OP is asking the difference between Level and Altitude. My answer (as it states) answer the question of that difference whether represented in metric meters or imperial feet.

In the original, unedited question (as you put it), user50042 never mentioned Flight Levels, so there is no evidence they understood that "Flight Level" is equivalent to "Standard Metric Level". All subsequent edits were done by different users (specifically, Pondlife and expeditedescent) so there's still no evidence that user50042 knew this. You can't just assume that the asker is going to know that the two terms are equivalent, especially with such a basic question.

@HiddenWindshield - Pondlife specifically added to the references to Flight Level to the question. The OP did not mention Flight Level. I answered the question as it was asked after Pondlife’s edit. That is the issue with edits. Unless you play detective with each question, you can’t be sure the edit has the original intent of the OP.

Yeah, that's true.


text:
Why are the leading edges on the Boeing 787 made from aluminum?
Why are the leading edges of the wings, horizontal stabiliser and vertical stabiliser and the engine's inlets made from Aluminium?

Because those surfaces are usually heated for anti-ice.

Because gold would be too expensive, too soft, and too heavy?

@mins:  And, of course, having a metal radome would defeat the purpose of a radome.

There are probably many factors in choosing aluminum vs. composites.
A big one is bird strikes. The leading edges are at the highest risk for this. Metal tends to absorb the impact better while retaining its strength, while composites would tend to delaminate and become much weaker. Resistance to in-flight hail must also be considered.

(Source Left, Right) Comparison of bird strike effect on aluminum and carbon fiber.
Anti ice was also mentioned. These areas are heated with bleed air (or, for the 787, electrical elements) to melt or prevent any ice formation. Metals do much better at high temperatures than composites, which would tend to melt or lose strength. Metals are also able to conduct the anti ice heat better than composites.
Another factor could be manufacturing. These surfaces have a lot of curvature. Composite panels are often reinforced with honeycomb core, which is best on relatively flat surfaces. Of course composites could be made into these shapes, but the shape would limit material and configuration options. Metal is also easier to work with or repair than composites.
As far as choosing aluminum over other metals, it tends to be very light and cheap for its strength, has decent temperature capability for anti-ice, and when properly used is fairly resistant to corrosion or fatigue.
The nose does not have anti ice and must serve as a cover for the weather radar. Composites can be made relatively transparent to radar, but metal would tend to block the waves.

WRT anti-ice, metals also conduct heat much better than typical composites, making your anti-ice heating more effective.

Use overexpanded Nomex cores, and curvature is not such a problem anymore. Don't let that distract you; the answer is absolutely correct.

The ease of repairing aluminium is also a lot easier than composites. Composites work best when the whole structure is made from a single piece and you don't want to replace such large sections every time you have a bird strike.

Curvature isn't an issue when making bicycle frames out of composite materials, and bikes have much tighter curves than the leading edge of a large airliner.

@DavidRicherby: But do bicycle frames use sandwich shells? The ones I know are either tubes with a single solid wall, or they use a light wall with a foam core over the full diameter. Curvature is a real problem for sandwich structures, especially when you have curvature in two directions.

@Notts90: May I disagree? The experience from the Beech Starship was that composite structures are much easier to repair than metal structures. All it needs is a skilled person doing the work (NC-35 crash in Denmark). Also, with much less corrosion or fatigue problems there is much less to repair in the first place. Only impact is worse; that's why leading edges are made from metal.

@PeterKämpf the joy of the internet is the that you certainly can! I completely agree regarding fatigue and rust. For impacts though, I was lead to believe that composites meant always replacing the whole panel where metal had other options (not 100% what they are).

There are a few reasons, the first is that the leading edge of the wing will be eroded in service by flying through dust, sand rain, hail etc. Metals are always typically used on such surfaces (termed an 'erosion shield') to reduce this effect, and also because minor damage can be polished out to keep the aircraft looking nice. Aluminum isn't the hardest waring material in this environment, but it is the lightest. Rotor blades would typically use Titanium or Nickel instead, as these harder metals work better in this more extreme environment.
The second reason is lightning strike; the leading edges of wings are common attachment points for lightning, so they are grounded to the airframes lightning return network. Making them from highly conductive metals can reduce the immediate damage caused as a result lightning attachment.

To make composites lightning-proof, a copper mesh is added to the layup. The real reason for metal leading edges is indeed impact tolerance. Once a leading edge is painted, erosion of that paint will not differ between metal and composite structures.

Aluminum is used for erosion prevention. There is no composite solution which can withstand the erosion caused by water/rain or FOD (sand/grit). In addition, aluminum leading edges transfer heat better than composites, which allows for improved anti-icing.